This invention relates generally to magnetic resonance imaging (MRI).
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials, and provides medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
MRI is based on nuclear spins, which can be viewed as vectors in a three-dimensional space. During an MRI process, each nuclear spin responds to four different effects: precession about the main magnetic field, nutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation. In steady-state MRI processes, a combination of these effects occurs periodically.
Gradient echo (GRE) sequences provide significantly improved acquisition speed, and have found wide use in numerous magnetic resonance imaging (MRI) applications such as cardiac imaging, angiography, and 3D volume imaging. However, because data are acquired while the magnetization is in a dynamic equilibrium known as steady-state, GRE demonstrates different signal characteristics than spin-echo sequences. Manipulating the flip angle of radio-frequency (RF) excitations to induce T1- or T2-weighting generally results in poor signal-to-noise ratio (SNR). Furthermore, the short repetition times (TR) of GRE sequences are not suitable for direct application of conventional contrast-generation methods (e.g., inversion recovery, T2-preparation) every TR.
Patents that are in the field of the invention are U.S. Pat. No. 5,122,747 by Riederer et al., issued Jun. 16, 1992, entitled “Spiral Three-Dimensional Fourier Transform NMR Scan,” U.S. Pat. No. 5,245,282 by Mugler et al., issued Sep. 14, 1993, entitled “Three-Dimensional Magnetic Resonance Imaging,” U.S. Pat. No. 5,912,557 by Wilman et al., issued Jun. 15, 1999, entitled “Centric Phase Encoding Order for 3D NMR Data Acquisition,” US H1,968 by Bernstein, published Jun. 5, 2001, entitled “Hyperpolarized MR Imaging Using Pulse Sequence with Progressively Increasing Flip Angle,” U.S. Pat. No. 6,885,193 by Foxall, issued Apr. 26, 2005, entitled “Magnetization Primer Sequence for Balanced Steady State Free Precision Imaging,” U.S. Pat. No. 7,646,924 by Donoho, issued Jan. 12, 2010, entitled “Method and Apparatus for Compressed Sensing,” U.S. Pat. No. 7,164,268 by Mugler et al., issued Jan. 16, 2007, entitled “Method and Apparatus for Spin-Echo-Train MR Imaging Using Prescribed Signal Evolutions,” and US Patent Publication No. 2008/0319301 by Busse, published Dec. 25, 2008, entitled “Method and Apparatus for Generating a Flip Angle Schedule for a Spin Echo Train Pulse Sequence,” all of which are incorporated by reference for all purposes.